Hi,
I found this way to make Caja showing openraster and .kra thumbnails, which is at least to me a very good point because 2 of my “main” softwares are Krita and Mypaint !
Maybe there ise another way but this works in Caja 1.16.2
open a terminal and copy and past those twa commands :
sudo add-apt-repository ppa:mmolch
press enter
enter password
say yes
sudo apt update && sudo apt install mmolch-thumbnailers
press enter
close terminal, open caja and you should be able to see .ora and .kra